Polysiloxanes

Polysiloxanes (PDMS, silicon oil, silicone, ...) are extremely surface active compounds. This makes them a desired component in many products such as coatings, adhesives or cosmetics. However, the surface activity is also a common reason for many wettability or adhesion failures which can be observed even for low Polysiloxane surface concentrations (surface density < 1 molecular layer).

Thus, in case of coating or adhesion problems a series of questions arise: 

Is a Polysiloxan present at a surface?

A detection of polysiloxanes is a straight forward exercise using ToF-SIMS. With detection limits in the ppm-range, ToF-SIMS is by far more sensitive than other established detection methods (e.g. IR spetroscopy). Therefore, even low but adhesion preventing amounts of polysiloxanes can be detected. 

How much Polysiloxane can my process tolerate?

A common strategy for answering this question is the combination of semi-quantitative ToF-SIMS data with suited adhesion tests (e.g. cross-cut adhesion test of coatings).

Figure1:Semi-quantitative comparison of the polysiloxane content and coating adhesion strength of different steel surfaces.

As an example figure 1 shows the correlation of normalised, semi-quantitative polysiloxane intensities of a steel surface (y-axis) and colour coded results of an adhesion test (x-axis; red: bad, green:good adhesion). Polysiloxanes were detected on all steel plates. Based on both, the ToF-SIMS and cross-cut adhesion analyses a critical polysiloxane value for the subsequent coating process was determined. Additionally, a Standard Operation Procedure (SOP; traffic light code) was established in order to determine the polysiloxane load of new steel charges prior to coating routinely. 

Figure 2: Distribution of different substances at the surface of a paint crater.

In addition to the already mentioned identification and quantification issues, ToF-SIMS is also capable to distinguish between different polysiloxane classes. Figure 2 illustrates such a difference by means of chemical maps ("images") of a paint crater. The coating contains a modified Polysiloxane B. In the crater region, this Polysiloxane is replaced by another Polysiloxane A. Additionally, a Poly­tetra­fluoro­ethylene (PTFE, Teflon) related particle is detected in the crater centre. Both, PTFE as well as the unadapted Polysiloxane A, modify the wettability characteristics of the coating which finally leads to a crater formation.